A powerful programming language is more than just a means for
instructing a computer to perform tasks. The language also serves as
a framework within which we organize our ideas about processes. Thus,
when we describe a language, we should pay particular attention to the
means that the language provides for combining simple ideas to form
more complex ideas. Every powerful language has three mechanisms for
accomplishing this:

primitive expressions, which represent the simplest
entities the language is concerned with,

means of combination, by which compound
elements are built from simpler ones, and

means of abstraction, by
which compound elements can be named and manipulated as units.

In programming, we deal with two kinds of elements: procedures and
data. (Later we will discover that they are really not so distinct.)
Informally, data is ``stuff'' that we want to manipulate, and
procedures are descriptions of the rules for manipulating the data.
Thus, any powerful programming language should be able to describe
primitive data and primitive procedures and should have methods for
combining and abstracting procedures and data.

In this chapter we will deal only with simple numerical data so that
we can focus on the rules for building procedures.4 In later chapters we will see that
these same rules allow us to build procedures to manipulate compound
data as well.

One easy way to get started at programming is to examine some typical
interactions with an interpreter for the Scheme dialect of Lisp.
Imagine that you are sitting at a computer terminal. You type an
expression, and the interpreter responds by displaying the result of
its evaluating that expression.

One kind of primitive expression you might type is a number. (More
precisely, the expression that you type consists of the numerals that
represent the number in base 10.) If you present Lisp with
a number

Expressions representing numbers may be combined with an expression
representing a primitive procedure (such as + or *) to form a
compound expression that represents the
application of the procedure to those numbers. For example:

(+ 137 349)486(- 1000 334)666
(* 5 99)495(/ 10 5)2
(+ 2.7 10)12.7

Expressions such as these, formed by delimiting a list of expressions
within parentheses in order to denote procedure application,
are called combinations. The leftmost
element in the list is called the operator, and the other
elements are called operands. The value of a combination is
obtained by applying the procedure specified by the operator to the
arguments that are the values of the operands.

The convention of placing the operator to the left of the operands is
known as prefix notation, and it may be somewhat confusing at
first because it departs significantly from the customary mathematical
convention. Prefix notation has several advantages, however. One of
them is that it can accommodate procedures that may take an arbitrary
number of arguments, as in the following examples:

(+ 21 35 12 7)75

(* 25 4 12)1200

No ambiguity can arise, because the operator is always the leftmost
element and the entire combination is delimited by the
parentheses.

A second advantage of prefix notation is that it extends in a
straightforward way to allow combinations to be nested, that is,
to have combinations whose elements are themselves
combinations:

(+ (* 3 5) (- 10 6))19

There is no limit (in principle) to the depth of such nesting and to
the overall complexity of the expressions that the Lisp interpreter
can evaluate.
It is we humans who get confused by still relatively
simple expressions such as

(+ (* 3 (+ (* 2 4) (+ 3 5))) (+ (- 10 7) 6))

which the interpreter would readily evaluate to be 57. We can help
ourselves by writing such an expression in the form

(+ (* 3
(+ (* 2 4)
(+ 3 5)))
(+ (- 10 7)
6))

following a formatting convention known as pretty-printing, in
which each long combination is written so that the operands are
aligned vertically. The resulting indentations display clearly the
structure of the expression.6

Even with complex expressions, the interpreter always operates in the
same basic cycle: It reads an expression from the terminal,
evaluates the expression, and prints the result.
This mode of operation is often expressed by saying that the
interpreter runs in a read-eval-print loop.
Observe in particular that it is not necessary to explicitly
instruct the interpreter to print the value of the expression.7

Define is our language's
simplest means of abstraction, for it allows us to use simple names to
refer to the results of compound operations, such as the
circumference computed above.
In general, computational objects may have very complex
structures, and it would be extremely inconvenient to have to remember
and repeat their details each time we want to use them. Indeed,
complex programs are constructed by building, step by step,
computational objects of increasing complexity. The
interpreter makes this step-by-step program construction particularly
convenient because name-object associations can be created
incrementally in successive interactions. This feature encourages the
incremental development and testing of programs and is largely
responsible for the fact that a Lisp program usually consists of a large
number of relatively simple procedures.

It should be clear that the possibility of associating values with
symbols and later retrieving them means that the interpreter must
maintain some sort of memory that keeps track of the name-object
pairs. This memory is called the environment (more precisely
the global environment, since we will see later that a
computation may involve a number of different
environments).9

One of our goals in this chapter is to isolate issues about thinking
procedurally. As a case in point, let us consider that, in evaluating
combinations, the interpreter is itself following a procedure.

To evaluate a combination, do the following:

1. Evaluate the subexpressions of the combination.

2. Apply the procedure that is the value of the leftmost
subexpression (the operator) to the arguments that are the values of
the other subexpressions (the operands).

Even this simple rule illustrates some important points about
processes in general. First, observe that the first step dictates
that in order to accomplish the evaluation process for a combination
we must first perform the evaluation process on each element of the
combination. Thus, the evaluation rule is recursive in nature;
that is, it includes, as one of its steps, the need to invoke the rule
itself.10

Notice how succinctly the idea of recursion can be used to express
what, in the case of a deeply nested combination, would otherwise be
viewed as a rather complicated process. For example, evaluating

(* (+ 2 (* 4 6))
(+ 3 5 7))

requires that the evaluation rule be applied to four different
combinations. We can obtain a picture of this process by representing
the combination in the form of a tree, as shown in
figure 1.1. Each combination is represented by a
node with branches corresponding to the operator and the
operands of the combination stemming from it.
The terminal nodes (that is, nodes with
no branches stemming from them) represent either operators or numbers.
Viewing evaluation in terms of the tree, we can imagine that the
values of the operands percolate upward, starting from the terminal
nodes and then combining at higher and higher levels. In general, we
shall see that recursion is a very powerful technique for dealing with
hierarchical, treelike objects. In fact, the ``percolate values
upward'' form of the evaluation rule is an example of a general kind
of process known as tree accumulation.

Figure 1.1: Tree representation, showing the value of each subcombination.

Next, observe that the repeated application of the first step brings
us to the point where we need to evaluate, not combinations, but
primitive expressions such as numerals, built-in operators, or other
names. We take care of the primitive cases by stipulating that

the values of numerals are the numbers that they name,

the values of built-in operators are the machine
instruction sequences that carry out the corresponding operations, and

the values of other names are the objects associated
with those names in the environment.

We may regard the second rule as a special case of the third one by
stipulating that symbols such as + and * are also included
in the global environment, and are associated with the sequences of
machine instructions that are their ``values.'' The key point to
notice is the role of the environment in determining the meaning of
the symbols in expressions. In an interactive language such as
Lisp, it is meaningless to speak of the value of an expression such as
(+ x 1) without specifying any information about the environment
that would provide a meaning for the symbol x (or even for the
symbol +). As we shall see in chapter 3, the general notion of
the environment as providing a context in which evaluation takes place
will play an important role in our understanding of program execution.

Notice that the
evaluation rule given above does not handle definitions.
For instance, evaluating
(define x 3) does not apply define to two arguments, one
of which is the value of the symbol x and the other of which is
3, since the purpose of the define is precisely to associate
x with a value.
(That is, (define x 3) is not a combination.)

Such exceptions to the general evaluation rule are called special
forms. Define is the only example of a special form that we
have seen so far, but we will meet others shortly. Each special form
has its own evaluation rule. The various kinds of expressions (each
with its associated evaluation rule) constitute the syntax of the
programming language. In comparison with most other programming
languages, Lisp has a very simple syntax; that is, the evaluation rule
for expressions can be described by a simple general rule together
with specialized rules for a small number of special
forms.11

We have here a compound procedure, which has been given the name
square. The procedure represents the operation of multiplying
something by itself. The thing to be multiplied is given a local
name, x, which plays the same role that a pronoun plays in
natural language. Evaluating the definition creates this
compound procedure and associates it with the name square.12

The general form of a procedure definition is

(define (<name> <formal parameters>) <body>)

The <name> is a symbol to be associated with the procedure
definition in the environment.13
The <formal parameters> are
the names used within the body of the procedure to refer to the
corresponding arguments of the procedure. The
<body> is an expression that will yield the value of
the procedure application when the formal parameters are replaced by
the actual arguments to which the procedure is applied.14
The <name>
and the <formal parameters>
are grouped within parentheses, just as they
would be in an actual call to the procedure being defined.

Having defined square, we can now use it:

(square 21)441

(square (+ 2 5))49

(square (square 3))81

We can also use square as a building block in defining other
procedures. For example, x2 + y2 can be expressed as

(+ (square x) (square y))

We can easily define a procedure
sum-of-squares that, given any two numbers as arguments, produces the
sum of their squares:

(define (sum-of-squares x y)
(+ (square x) (square y)))

(sum-of-squares 3 4)25

Now we can use sum-of-squares as a building block in constructing
further procedures:

(define (f a)
(sum-of-squares (+ a 1) (* a 2)))

(f 5)136

Compound procedures are used in exactly the same way as primitive
procedures. Indeed, one could not tell by looking at the definition
of sum-of-squares given above whether square was built into
the interpreter, like + and *, or defined as a compound procedure.

To evaluate a combination whose operator names a compound procedure, the
interpreter follows much the same process as for combinations whose
operators name primitive procedures, which we described in
section 1.1.3. That is, the interpreter
evaluates the elements of the combination and applies the procedure
(which is the value of the operator of the combination) to the
arguments (which are the values of the operands of the combination).

We can assume that the mechanism for applying primitive procedures to
arguments is built into the interpreter. For compound procedures, the
application process is as follows:

To apply a compound procedure to arguments, evaluate the body of the
procedure with each formal parameter replaced by the corresponding
argument.

To illustrate this process, let's evaluate the combination

(f 5)

where f is the procedure defined in
section 1.1.4. We begin by retrieving the
body of f:

(sum-of-squares (+ a 1) (* a 2))

Then we replace the formal parameter a by the argument 5:

(sum-of-squares (+ 5 1) (* 5 2))

Thus the problem reduces to the evaluation of a combination with two
operands and an operator sum-of-squares. Evaluating this
combination involves three subproblems. We must evaluate the
operator to get the procedure to be applied, and we must evaluate the
operands to get the arguments. Now (+ 5 1) produces 6 and
(* 5 2) produces 10, so we must apply the
sum-of-squares procedure to 6 and 10. These values are substituted
for the formal parameters x and y in the body of sum-of-squares,
reducing the expression to

(+ (square 6) (square 10))

If we use the definition of square, this reduces to

(+ (* 6 6) (* 10 10))

which reduces by multiplication to

(+ 36 100)

and finally to

136

The process we have just described is called the substitution
model for procedure application. It can be taken as a model that
determines the ``meaning'' of procedure application, insofar as the
procedures in this chapter are concerned. However, there are two
points that should be stressed:

The purpose of the substitution is to help us think about
procedure application, not to provide a description of how
the interpreter really works. Typical interpreters do not evaluate
procedure applications by manipulating the text of a procedure to
substitute values for the formal parameters. In practice, the
``substitution'' is accomplished by using a local environment for the
formal parameters. We will discuss this more fully in chapters 3 and
4 when we examine the implementation of an interpreter in detail.

Over the course of this book, we will present a sequence of
increasingly elaborate models of how interpreters work, culminating
with a complete implementation of an interpreter and compiler in
chapter 5. The substitution model is only the first of these
models -- a way to get started thinking formally about the evaluation
process. In general, when modeling phenomena in science and
engineering, we begin with simplified, incomplete models. As we
examine things in greater detail, these simple models become
inadequate and must be replaced by more refined models. The
substitution model is no exception. In particular, when we address in
chapter 3 the use of procedures with ``mutable data,'' we will see that
the substitution model breaks down and must be replaced by a more
complicated model of procedure application.15

According to the description of evaluation given in
section 1.1.3, the interpreter first
evaluates the operator and operands and then applies the resulting procedure
to the resulting arguments. This is not the only way to perform
evaluation. An alternative evaluation model would not evaluate the
operands until their values were needed. Instead it would first substitute
operand expressions for parameters until
it obtained an expression involving only primitive operators, and
would then perform the evaluation. If we used this method, the
evaluation of

(f 5)

would proceed according to the sequence of expansions

(sum-of-squares (+ 5 1) (* 5 2))

(+ (square (+ 5 1)) (square (* 5 2)) )

(+ (* (+ 5 1) (+ 5 1)) (* (* 5 2) (* 5 2)))

followed by the reductions

(+ (* 6 6) (* 10 10))

(+ 36 100)

136

This gives the same answer as our previous evaluation model, but the
process is different. In particular, the evaluations
of (+ 5 1) and (* 5 2) are each performed twice here,
corresponding to the reduction of the expression

(* x x)

with x replaced respectively by (+ 5 1) and (* 5 2).

This alternative ``fully expand and then reduce'' evaluation method is
known as normal-order evaluation, in contrast to the ``evaluate
the arguments and then apply'' method that the interpreter actually
uses, which is called applicative-order evaluation. It can be
shown that, for procedure applications that can be modeled using
substitution (including all the procedures in the first two chapters
of this book) and that yield legitimate values, normal-order and
applicative-order evaluation produce the same value. (See
exercise 1.5 for an instance of
an ``illegitimate'' value where normal-order and applicative-order
evaluation do not give the same result.)

Lisp uses applicative-order evaluation, partly because of the
additional efficiency obtained from avoiding multiple evaluations of
expressions such as those illustrated with (+ 5 1) and (* 5
2) above and, more significantly, because normal-order evaluation
becomes much more complicated to deal with when we leave the realm of
procedures that can be modeled by substitution. On the other hand,
normal-order evaluation can be an extremely valuable tool, and we will
investigate some of its implications in chapters 3 and 4.16

The expressive power of the class of procedures that we can define at
this point is very limited, because we have no way to make tests and
to perform different operations depending on the result of a test.
For instance, we cannot define a procedure that computes the absolute
value of a number by testing whether the number is positive, negative,
or zero and taking different actions in the different cases according
to the rule

This construct is called a case analysis, and
there is a special form in Lisp for notating such a case
analysis. It is called cond (which stands for
``conditional''), and it is used as follows:

(define (abs x)
(cond ((> x 0) x)
((= x 0) 0)
((< x 0) (- x))))

The general form of a conditional expression is

(cond (<p1> <e1>)
(<p2> <e2>)
(<pn> <en>))

consisting of the symbol cond followed by
parenthesized pairs of expressions (<p> <e>)
called clauses. The first expression in each pair is a predicate -- that is, an expression whose value is interpreted as
either true or false.17

Conditional expressions are evaluated as follows. The predicate
<p1> is evaluated first. If its value is false, then
<p2> is evaluated. If <p2>'s value is also false, then
<p3> is evaluated. This process continues until a predicate is
found whose value is true, in which case the interpreter returns the
value of the corresponding consequent expression <e> of the
clause as the value of the conditional expression. If none of the
<p>'s is found to be true, the value of the cond is
undefined.

The word predicate is used for procedures that return true
or false, as well as for expressions that evaluate to true or false.
The absolute-value procedure abs makes use of the primitive
predicates >, <, and =.18 These take two
numbers as arguments and test whether the first number is,
respectively, greater than, less than, or equal to the second number,
returning true or false accordingly.

Another way to write the absolute-value procedure is

(define (abs x)
(cond ((< x 0) (- x))
(else x)))

which could be expressed in English as ``If x is less than zero
return - x; otherwise return x.'' Else is a special symbol
that can be used in place of the <p> in the final clause of a cond. This causes the cond to return as its value the value of
the corresponding <e> whenever all previous clauses have been
bypassed. In fact, any expression that always evaluates to a true
value could be used as the <p> here.

Here is yet another way to write the absolute-value procedure:

(define (abs x)
(if (< x 0)
(- x)
x))

This uses the special form if, a restricted type of conditional
that can be used when there are precisely two cases in the case
analysis. The general form of an if expression is

(if <predicate> <consequent> <alternative>)

To evaluate an if expression, the interpreter starts by evaluating
the <predicate> part of the expression. If the <predicate>
evaluates to a true value, the interpreter then evaluates
the <consequent> and returns its value. Otherwise it evaluates
the <alternative> and returns its value.19

In addition to primitive
predicates such as <, =, and >, there are logical
composition operations, which enable us to construct compound
predicates. The three most frequently used are these:

(and <e1> ... <en>)

The interpreter
evaluates the expressions <e> one at a time, in left-to-right order. If
any <e> evaluates to false, the value of the and
expression is false, and the rest of the <e>'s are not evaluated.
If all <e>'s evaluate to true values, the value of the and
expression is the value of the last one.

(or <e1> ... <en>)

The interpreter
evaluates the expressions <e> one at a time, in left-to-right order. If
any <e> evaluates to a true value, that value is
returned as the value of the or expression,
and the rest of the <e>'s are not evaluated.
If all <e>'s evaluate to false,
the value of the or expression is false.

(not <e>)

The value of a not expression is true
when the expression <e> evaluates to false, and false otherwise.

Notice that and and or are special forms, not procedures,
because the subexpressions are not necessarily all evaluated.
Not is an ordinary procedure.

As an example of how these are used, the condition that a number x
be in the range 5 < x < 10 may be expressed as

(and (> x 5) (< x 10))

As another example, we can define a predicate to test whether one
number is greater than or equal to another as

(define (>= x y)
(or (> x y) (= x y)))

or alternatively as

(define (>= x y)
(not (< x y)))

Exercise 1.1. Below is a sequence of expressions.
What is the result printed by the interpreter in response to each
expression? Assume that the sequence is to be evaluated in the order
in which it is presented.

Exercise 1.3. Define a procedure that takes three numbers as arguments and returns
the sum of the squares of the two larger numbers.

Exercise 1.4.Observe that our model of evaluation allows for combinations whose
operators are compound expressions. Use this observation to
describe the behavior of the following procedure:

(define (a-plus-abs-b a b)
((if (> b 0) + -) a b))

Exercise 1.5.Ben Bitdiddle has invented a test to determine whether the interpreter
he is faced with is using applicative-order evaluation or normal-order
evaluation. He defines the following two procedures:

(define (p) (p))

(define (test x y)
(if (= x 0)
0
y))

Then he evaluates the expression

(test 0 (p))

What behavior will Ben observe with an interpreter that uses
applicative-order evaluation? What behavior will he observe with an
interpreter that uses normal-order evaluation? Explain your answer.
(Assume that the evaluation rule for the special form if is the
same whether the interpreter is using normal or applicative order:
The predicate expression is evaluated first, and the result
determines whether to evaluate
the consequent or the alternative expression.)

Procedures, as introduced above, are much like ordinary mathematical
functions. They specify a value that is determined by one or more
parameters. But there is an important difference between
mathematical functions and computer procedures. Procedures must be
effective.

As a case in point, consider the problem of computing square
roots. We can define the square-root function as

This describes a perfectly legitimate mathematical function. We could
use it to recognize whether one number is the square root of another, or
to derive facts about square roots in general. On the other hand, the
definition does not describe a procedure. Indeed, it tells us almost
nothing about how to actually find the square root of a given number. It
will not help matters to rephrase this definition in pseudo-Lisp:

(define (sqrt x)
(the y (and (>= y 0)
(= (square y) x))))

This only begs the question.

The contrast between function and procedure is a reflection of the
general distinction between describing properties of things and
describing how to do things, or, as it is sometimes referred to, the
distinction between declarative knowledge and imperative knowledge.
In mathematics we are usually concerned with declarative (what is)
descriptions, whereas in computer science we are usually concerned
with imperative (how to) descriptions.20

How does one compute square roots? The most common way is to use
Newton's method of successive approximations, which says that whenever
we have a guess y for the value of the square root of a number x,
we can perform a simple manipulation to get a better guess (one closer
to the actual square root) by averaging y with
x/y.21 For example, we can compute
the square root of 2 as follows. Suppose our initial guess is 1:

Guess

Quotient

Average

1

(2/1) = 2

((2 + 1)/2) = 1.5

1.5

(2/1.5) = 1.3333

((1.3333 + 1.5)/2) = 1.4167

1.4167

(2/1.4167) = 1.4118

((1.4167 + 1.4118)/2) = 1.4142

1.4142

...

...

Continuing this process, we obtain better and better
approximations to the square root.

Now let's formalize the process in terms of procedures. We start with
a value for the radicand (the number whose square root we are trying
to compute) and a value for the guess. If the guess is good enough
for our purposes, we are done; if not, we must repeat the process with an
improved guess. We write this basic strategy as a procedure:

A guess is improved by averaging
it with the quotient of the radicand and the old guess:

(define (improve guess x)
(average guess (/ x guess)))

where

(define (average x y)
(/ (+ x y) 2))

We also have to say what we mean by ``good enough.'' The
following will do for illustration, but it is not really a very good
test. (See exercise 1.7.)
The idea is to improve the answer until it is close enough so that its
square differs from the radicand by less than a predetermined
tolerance (here 0.001):22

(define (good-enough? guess x)
(< (abs (- (square guess) x)) 0.001))

Finally, we need a way to get started. For instance, we can
always guess that the square root of any number is 1:23

(define (sqrt x)
(sqrt-iter 1.0 x))

If we type these definitions to the interpreter, we can use sqrt
just as we can use any procedure:

The sqrt program also illustrates that the simple procedural
language we have introduced so far is sufficient for writing any
purely numerical program that one could write in, say, C or
Pascal. This might seem surprising, since we have not included in
our language any iterative (looping) constructs that direct the
computer to do something over and over again. Sqrt-iter, on the
other hand, demonstrates how iteration can be accomplished using no
special construct other than the ordinary ability to call a
procedure.24

Exercise 1.6.Alyssa P. Hacker doesn't see why if needs
to be provided as a special form. ``Why can't I just define it as an
ordinary procedure in terms of cond?'' she asks.
Alyssa's friend Eva Lu Ator claims this can indeed be done, and
she defines a new version of if:

What happens when Alyssa attempts to use this to compute square roots?
Explain.

Exercise 1.7. The good-enough? test used in computing square roots will not be
very effective for finding the square roots of very small numbers.
Also, in real computers, arithmetic operations are almost always
performed with limited precision. This makes our test inadequate for
very large numbers. Explain these statements, with examples showing
how the test fails for small and large numbers. An alternative
strategy for implementing good-enough? is to watch how guess changes from one iteration to the next and to stop when the
change is a very small fraction of the guess. Design a square-root
procedure that uses this kind of end test. Does this work better for
small and large numbers?

Exercise 1.8.Newton's method for cube roots is based on the fact that if y is an
approximation to the cube root of x, then a better approximation is
given by the value

Use this formula to implement a cube-root procedure analogous to the
square-root procedure. (In section 1.3.4 we
will see how to implement Newton's method in general as an abstraction
of these square-root and cube-root procedures.)

Sqrt is our first example of a process defined by a set of
mutually defined procedures. Notice that the definition of sqrt-iter is recursive; that is, the procedure is defined in
terms of itself. The idea of being able to define a procedure in
terms of itself may be disturbing; it may seem unclear how such a
``circular'' definition could make sense at all, much less specify a
well-defined process to be carried out by a computer. This will be
addressed more carefully in
section 1.2. But first let's consider
some other important points illustrated by the sqrt example.

Observe that the problem of computing square roots breaks up naturally
into a number of subproblems: how to tell whether a guess is good
enough, how to improve a guess, and so on. Each of these tasks is
accomplished by a separate procedure. The entire sqrt program
can be viewed as a cluster of procedures (shown in
figure 1.2) that mirrors the decomposition of
the problem into subproblems.

Figure 1.2: Procedural decomposition of the sqrt program.

The importance of this decomposition strategy is not simply that one
is dividing the program into parts. After all, we could take any
large program and divide it into parts -- the first ten lines, the next
ten lines, the next ten lines, and so on. Rather, it is crucial that
each procedure accomplishes an identifiable task that can be used as a
module in defining other procedures. For example, when we define the
good-enough? procedure in terms of square, we are able to
regard the square procedure as a ``black box.'' We are not at
that moment concerned with how the procedure computes its
result, only with the fact that it computes the square. The details
of how the square is computed can be suppressed, to be considered at a
later time. Indeed, as far as the good-enough? procedure is
concerned, square is not quite a procedure but rather an
abstraction of a procedure, a so-called procedural abstraction.
At this level of abstraction, any procedure that computes the square
is equally good.

Thus, considering only the values they return, the following two procedures for
squaring a number should be indistinguishable. Each takes a numerical
argument and produces the square of that number as the
value.25

(define (square x) (* x x))

(define (square x)
(exp (double (log x))))

(define (double x) (+ x x))

So a procedure definition should be able to suppress detail. The
users of the procedure may not have written the procedure themselves,
but may have obtained it from another programmer as a black box. A
user should not need to know how the procedure is implemented in order
to use it.

One detail of a procedure's implementation that should not matter to
the user of the procedure is the implementer's choice of names for the
procedure's formal parameters. Thus, the following procedures should
not be distinguishable:

(define (square x) (* x x))

(define (square y) (* y y))

This principle -- that the meaning of a procedure should be independent
of the parameter names used by its author -- seems on the surface to be
self-evident, but its consequences are profound. The simplest
consequence is that the parameter names of a procedure must be local
to the body of the procedure. For example, we used square in
the definition of good-enough? in our square-root procedure:

(define (good-enough? guess x)
(< (abs (- (square guess) x)) 0.001))

The intention of the author of good-enough? is to determine if
the square of the first argument is within a given tolerance of the
second argument. We see that the author of good-enough? used
the name guess to refer to the first argument and x to
refer to the second argument. The argument of square is guess. If the author of square used x (as above)
to refer to that argument, we see that the x in good-enough? must be a different x than the one in square. Running the procedure square must not affect the value
of x that is used by good-enough?, because that value of
x may be needed by good-enough? after square is done
computing.

If the parameters were not local to the bodies of their respective
procedures, then the parameter x in square could be
confused with the parameter x in good-enough?, and the
behavior of good-enough? would depend upon which version of
square we used. Thus, square would not be the black box
we desired.

A formal parameter of a procedure has a very special role in the
procedure definition, in that it doesn't matter what name the formal
parameter has. Such a name is called a bound variable, and we
say that the procedure definition binds its formal parameters.
The meaning of a procedure definition is unchanged if a bound variable
is consistently renamed throughout the definition.26 If a variable is not bound, we say that it is free. The
set of expressions for which a binding defines a name is called the
scope of that name.
In a procedure definition, the bound variables
declared as the formal parameters of the procedure have the body of
the procedure as their scope.

In the definition of good-enough? above, guess and x are
bound variables but <, -, abs, and square are free.
The meaning of good-enough? should be independent of the names we
choose for guess and x so long as they are distinct and
different from <, -, abs, and square. (If we renamed
guess to abs we would have introduced a bug by capturing
the variable abs. It would have changed from free to bound.) The
meaning of good-enough? is not independent of the names of its
free variables, however. It surely depends upon the fact (external to
this definition) that the symbol abs names a procedure for
computing the absolute value of a number. Good-enough? will
compute a different function if we substitute cos for abs in
its definition.

We have one kind of name isolation available to us so far: The formal
parameters of a procedure are local to the body of the procedure. The
square-root program illustrates another way in which we would like to
control the use of names. The existing program consists of
separate procedures:

The problem with this program is that the only procedure that is
important to users of sqrt is sqrt. The other
procedures (sqrt-iter, good-enough?, and improve)
only clutter up their minds. They may not define any other procedure
called good-enough? as part of another program to work together
with the square-root program, because sqrt needs it. The
problem is especially severe in the construction of large systems by
many separate programmers. For example, in the construction of a
large library of numerical procedures, many numerical functions are
computed as successive approximations and thus might have procedures
named good-enough? and improve as auxiliary procedures.
We would like to localize the subprocedures, hiding them inside sqrt so that sqrt could coexist with other successive
approximations, each having its own private
good-enough? procedure. To make this possible, we allow a
procedure to have
internal definitions that are local to that procedure. For example,
in the square-root problem we can write

Such nesting of definitions, called block structure,
is basically the right solution to the simplest
name-packaging problem. But there is a better idea lurking here. In
addition to internalizing the definitions of the auxiliary procedures,
we can simplify them. Since x is bound in the definition of
sqrt, the procedures good-enough?, improve, and
sqrt-iter, which are defined internally to sqrt, are in the
scope of x. Thus, it is not necessary to pass x explicitly to
each of these procedures. Instead, we allow x to be a free
variable in the internal definitions, as shown below. Then x
gets its value from the argument with which the enclosing
procedure sqrt is called. This discipline is called lexical
scoping.27

We will use block structure extensively to help us break
up large programs into tractable pieces.28
The idea of block structure originated with the
programming language Algol 60. It appears in most advanced
programming languages and is an important tool for helping to organize
the construction of large programs.

4 The
characterization of numbers as ``simple data'' is a barefaced bluff.
In fact, the treatment of numbers is one of the trickiest and most
confusing aspects of any programming language. Some typical issues
involved are these:
Some computer systems distinguish integers, such as 2, from
real numbers, such as 2.71. Is the real number 2.00 different
from the integer 2?
Are the arithmetic operations used
for integers the same as the operations used for real numbers? Does 6
divided by 2 produce 3, or 3.0? How large a number can we represent?
How many decimal places of accuracy can we represent? Is the range of
integers the same as the range of real numbers? Above and beyond
these questions, of course, lies a collection of issues concerning
roundoff and truncation errors -- the entire science of numerical
analysis. Since our focus in this book is on large-scale program
design rather than on numerical techniques, we are going to ignore
these problems. The numerical examples in this chapter will exhibit
the usual roundoff behavior that one observes when using arithmetic
operations that preserve a limited number of decimal places of
accuracy in noninteger operations.

5 Throughout this book,
when we wish to emphasize the distinction between the input typed by
the user and the response printed by the interpreter, we will show the
latter in slanted characters.

6Lisp systems typically provide
features to aid the user in formatting expressions. Two especially
useful features are one that automatically indents to the proper
pretty-print position whenever a new line is started and one that
highlights the matching left parenthesis whenever a right parenthesis
is typed.

7Lisp obeys the convention that every
expression has a value. This convention, together with the old
reputation of Lisp as an inefficient language, is the source of the
quip by Alan Perlis (paraphrasing Oscar Wilde)
that ``Lisp programmers know the value of
everything but the cost of nothing.''

8In this book, we do not show the interpreter's response to
evaluating definitions, since this is highly
implementation-dependent.

9 Chapter 3 will show that this notion of
environment is crucial, both for understanding how the interpreter
works and for implementing interpreters.

10 It may seem strange that the evaluation rule says, as
part of the first step, that we should evaluate the leftmost element
of a combination, since at this point that can only be an operator
such as + or *
representing a built-in primitive procedure such as addition or
multiplication. We will see later that it is useful to be able to work with
combinations whose operators are themselves compound expressions.

11Special syntactic forms that are simply convenient
alternative surface structures for things that can be written in more
uniform ways are sometimes called syntactic sugar, to use a
phrase coined by Peter Landin. In comparison with users of other
languages, Lisp programmers, as a rule, are less concerned with
matters of syntax. (By contrast, examine any Pascal manual and notice
how much of it is devoted to descriptions of syntax.) This disdain
for syntax is due partly to the flexibility of Lisp, which makes it
easy to change surface syntax, and partly to the observation that many
``convenient'' syntactic constructs, which make the language less
uniform, end up causing more trouble than they are worth when programs
become large and complex. In the words of Alan Perlis, ``Syntactic
sugar causes cancer of the semicolon.''

12 Observe that there are two different operations
being combined here: we are creating the procedure, and we are giving
it the name square. It is possible, indeed important, to be
able to separate these two notions -- to create procedures without
naming them, and to give names to procedures that have already been
created. We will see how to do this in section 1.3.2.

13 Throughout this book, we will
describe the general syntax of expressions by using italic symbols
delimited by angle brackets -- e.g., <name> -- to denote the
``slots'' in the expression to be filled in when such an expression is
actually used.

14 More
generally, the body of the procedure can be a sequence of expressions.
In this case, the interpreter evaluates each expression in the
sequence in turn and returns the value of the final expression as the
value of the procedure application.

15 Despite the
simplicity of the substitution idea, it turns out to be surprisingly
complicated to give a rigorous mathematical definition of the
substitution process. The problem arises from the possibility of
confusion between the names used for the formal parameters of a
procedure and the (possibly identical) names used in the expressions
to which the procedure may be applied. Indeed, there is a long
history of erroneous definitions of substitution in the
literature of logic and programming semantics. See Stoy 1977 for a
careful discussion of substitution.

16 In
chapter 3 we will introduce stream processing, which is a way of
handling apparently ``infinite'' data structures by incorporating a
limited form of normal-order evaluation. In
section 4.2 we will modify the Scheme
interpreter to produce a normal-order variant of Scheme.

17``Interpreted as either true or false''
means this: In Scheme, there are two distinguished values that are
denoted by the constants #t and #f. When the interpreter
checks a predicate's value, it interprets #f as false. Any other value
is treated as true. (Thus, providing #t is logically
unnecessary, but it is convenient.) In this book we will use
names true and false, which are associated
with the values #t and #f respectively.

18Abs also uses
the ``minus'' operator -, which, when used with a single
operand, as in (- x), indicates negation.

19 A minor difference
between if and cond is that the
<e> part of each cond clause may be a sequence of expressions.
If the corresponding <p> is found to be true, the expressions
<e> are evaluated in sequence and the value of the final
expression in the sequence is returned as the value of the cond.
In an if expression, however, the <consequent> and
<alternative> must be single expressions.

20 Declarative and
imperative descriptions are intimately related, as indeed are
mathematics and computer science. For instance, to say that the
answer produced by a program is ``correct'' is to make a declarative
statement about the program. There is a large amount of research
aimed at establishing techniques for proving that programs are
correct, and much of the technical difficulty of this subject has to
do with negotiating the transition between imperative statements (from
which programs are constructed) and declarative statements (which can
be used to deduce things). In a related vein, an important current
area in programming-language design is the exploration of so-called
very high-level languages, in which one actually programs in terms of
declarative statements. The idea is to make interpreters
sophisticated enough so that, given ``what is'' knowledge specified by
the programmer, they can generate ``how to'' knowledge automatically.
This cannot be done in general, but there are important areas where
progress has been made. We shall revisit this idea in chapter 4.

21 This square-root algorithm is actually a special case
of Newton's method, which is a general technique for finding roots of
equations. The square-root algorithm itself was developed by Heron of
Alexandria in the first century A.D. We will see how to express
the general Newton's method as a Lisp procedure in
section 1.3.4.

22 We will usually give
predicates names ending with question marks, to help us
remember that they are predicates. This
is just a stylistic convention. As far as the interpreter is
concerned, the question mark is just an ordinary character.

23 Observe that we express our initial guess as 1.0 rather than
1. This would not make any difference in many Lisp implementations.
MIT Scheme, however, distinguishes between exact integers and decimal
values, and dividing two integers produces a rational number rather
than a decimal. For example, dividing 10 by 6 yields 5/3, while
dividing 10.0 by 6.0 yields 1.6666666666666667. (We will learn how to
implement arithmetic on rational numbers in
section 2.1.1.) If we start with an initial guess of 1
in our square-root program, and x is an exact integer, all
subsequent values produced in the square-root computation will be
rational numbers rather than decimals. Mixed operations on rational
numbers and decimals always yield decimals, so starting with an
initial guess of 1.0 forces all subsequent values to be decimals.

24 Readers who are worried about the efficiency
issues involved in using procedure calls to implement iteration should
note the remarks on ``tail recursion'' in
section 1.2.1.

25 It is not even clear which of these procedures is a
more efficient implementation. This depends upon the hardware
available. There are machines for which the ``obvious''
implementation is the less efficient one. Consider a machine that has
extensive tables of logarithms and antilogarithms stored in a very
efficient manner.

26 The
concept of consistent renaming is actually subtle and difficult to
define formally. Famous logicians have made embarrassing errors
here.

27 Lexical
scoping dictates that free variables in a procedure are taken to refer to
bindings made by enclosing procedure definitions;
that is, they are looked up in
the environment in which the procedure was defined. We will see how
this works in detail in chapter 3 when we study environments and the
detailed behavior of the interpreter.

28 Embedded definitions
must come first in a procedure body. The management is not responsible
for the consequences of running programs that intertwine definition
and use.